This invention relates generally to projection displays.
A projection display system typically includes one or more spatial light modulators (SLMs) that modulate light for purposes of producing a projected image. The SLM may include, for example, a liquid crystal display (LCD) such as a high temperature polysilicon (HTPS) LCD panel or a liquid crystal on silicon (LCOS) microdisplay, a grating light valve or a MEMs (where “MEMs” stands for micro-electro-mechanical devices) light modulator such as a digital mirror display (DMD) to modulate light that originates from a lamp of the projection display system.
In typical projection display systems, the lamp output is formatted with optics to deliver a uniform illumination level on the surface of the SLM. The SLM forms a pictorial image by modulating the illumination into spatially distinct tones ranging from dark to bright based on supplied video data. Additional optics then relay and magnify the modulated illumination pattern onto a screen for viewing.
The SLM typically includes an array of pixel cells, each of which is electrically controllable to establish the intensity of a pixel of the projected image. In some projection display systems, SLMs are transmissive and in others, they are reflective. For the purposes of simplification, the discussion will address reflective SLMs.
An SLM may be operated in an analog manner by applying analog voltages to each pixel to effect a range of projected pixel brightnesses from black to grey to white. An SLM may also be operated in a digital manner so that each pixel has only two states: a default reflective state which causes either a bright or a dark projected pixel and a non-default reflective state which causes the opposite projected pixel intensity. In the case of an LCOS SLM, the pre-alignment orientation of the liquid crystal material and any retarders in the system determine whether the default reflective state is normally bright or normally dark. For the purposes of simplification, the discussion will denote the default reflective state as normally bright, i.e., one in which the pixel cell reflects incident light into the projection lens (the light that forms the projected image) to form a corresponding bright pixel of the projected image. Thus, in its basic operation, the pixel cell may be digitally-controlled to form either a dark pixel (in its non-default reflective state) or a bright pixel (in its default reflective state). In the case of a DLP SLM, the states may represent the pixel in a co-planar position to the underlying substrate.
Although its pixels are operated digitally, the above-described digitally-driven SLM may also be used in an application to produce visually perceived pixel intensities (called “gray scale intensities”) between the dark and bright levels. For such an application, each pixel may be controlled by pulse width modulation (PWM), a control scheme that causes the human eye to perceive gray scale intensities in the projected image, although each pixel cell still only assumes one of two states at any one time. The human visual system perceives a temporal average of pixel intensity when the PWM control operates at sufficiently fast rates.
In the PWM control scheme, a pixel intensity (or tone) is established by controlling the time that the pixel cell stays in its reflective state and the time that the pixel cell remains in the non-reflective state during an interval time called a PWM cycle. This type of control is also referred to as duty cycle control in that the duty cycle (the ratio of the time that the pixel cell is in its reflective state to the total time the pixel cell is in its non-reflective and reflective states) of each PWM cycle is controlled to set the pixel intensity. A relatively bright pixel intensity is created by having the pixel cell spend a predominant proportion of time in its reflective state during the PWM cycle, while a relatively dark pixel intensity is created by having the pixel cell spend a predominant amount of time in its non-reflective state during the PWM cycle.
Projection displays with single microdisplay panels that serve all three primary colors may be desirable to meet mass market price targets for large screen, high definition, televisions. Known single-panel display systems suffer from brightness losses and/or visual artifacts that consumers may find objectionable. For example, single-panel light engines may time share the single panel for red, green, and blue images, while illumination is sequentially modulated by means of a color wheel or spinning prism. For example, with a color wheel, with green data displayed, green illumination is applied to the panel. With blue data displayed, blue illumination is applied to the panel. With red data displayed, red illumination is applied to the panel. In a scrolling prism system, all three narrow color strips of red, green, and blue illumination move down or across the panel. The data must be synchronized to display the correct data for the color of impinging illumination.
Another approach for single-panel light engines uses microdisplay panels illuminated with white light, but integral to the panel are pixel-sized filters or diffractive or dispersive elements in a per pixel pattern to segregate incoming light by wavelength. In such systems, each pixel modulates light from a single color band, either red or green or blue.
Using time sequential illumination by red, green, and blue light may be subject to limitations, depending on whether the system is modulated by a color wheel or a rotating prism. If the illumination is modulated by the color wheel, the system brightness may suffer because only one-third of the illumination wavelengths are passed by the color wheel to impinge on the SLM Further, during periods when the color spoke transitions through the illuminating beam, the panels must be held in their dark state. When this is not done, the display does not achieve full saturation in each of the primary colors. Together, these two effects significantly reduce the brightness of a colorwheel based system. In the rotating prism approach, the illumination is modulated by color prefiltering and then bands of red, green, and blue light are scrolled by the rotation of the prism. Thus, all wavelengths of the illumination source pass through the prism onto the SLM. However, some rows or columns of a scrolling panel must also be held dark where the colors transition. Thus, the overall reflecting surface is reduced. Overall, color sequential systems may be less bright than non-temporal systems.
Further, color sequential illumination may cause visual artifacts. These artifacts are known as color breakup artifacts and are the result of an object of interest moving across the screen and being imaged by the viewer's eye. If the eye and the object have relative motion, the subsequent retinal images do not overlap spatially. Instead, there will be a motion displaced blue image, then a motion displaced red image, then a motion displaced green image. The eye does not fuse the three color records in such cases and color break up is perceived. The image can still exhibit color breakup in video systems when color fields are sequentially updated as rapidly as 2000 Hertz.
Thus, there is a need for better ways to make improved single-panel projection displays.